The present invention relates to rotary assemblies comprising a rotor mounted to rotate in a stator about a mean axis of rotation, and in which there is a need to detect the radial position of the rotor within the stator.
Detecting the radial position of the rotor makes it possible, for example, to detect wear in the bearings carrying the rotor, as described in document U.S. Pat. No. 4,199,718 A. In that document, the detector for detecting the radial position of the rotor comprises three sensors distributed in the stator at substantially 120xc2x0 from one another around the rotor and at equal distances from the mean axis of rotation of the rotor. Each sensor is a coil which produces an electric voltage at its terminals as a function of a magnetic field generated by a plurality of magnetic poles carried by the rotor. The coils are connected in series so as to generate a periodic voltage signal which, on processing, enables the radial position of the rotor within the stator to be evaluated. That structure requires the rotor to be provided with a multi-pole segment for generating the magnetic fields that are picked up by the coils. Although suitable for monitoring wear in mechanical bearings, that apparatus would not be suitable for controlling magnetic bearings in which supporting electromagnets generate a magnetic field that would interfere with the operation of the sensors.
Document CH 251 155 A describes apparatus for monitoring concentricity of a wire in its coating. A magnetic circuit having three radial branches in a star configuration interconnected by a peripheral ring carries three coils that are star-connected to a common connection point. The coils are fed from a three-phase voltage source having a neutral connection. A voltage sensor, connected between neutral and the common connection point, delivers a signal which is proportional to the amplitude of the eccentricity of the wire. That apparatus does not measure and could not be used for measuring the instantaneous radial position of a rotary shaft.
For the purpose of controlling a magnetic bearing, a detector is known for detecting the radial position of a rotor as described in document U.S. Pat. No. 4,114,960 A, which detector has at least two pairs of inductive sensors. The sensors in one pair of sensors are placed in positions that are diametrally opposite about the rotor, and they are electrically connected in series to be fed with a sinewave voltage. The output terminal from the sensor is connected to the point where the sensors are interconnected, and produces an electrical signal that is the image of rotor displacement along the diametral direction between the two sensors. Detecting the radial position of the rotor in any direction around the mean axis of rotation requires at least two pairs of sensors, e.g. directed along two perpendicular diameters.
The problem proposed by the invention is that of designing a novel detector structure for detecting the radial position of the rotor in the stator, using a small number of sensors and using sensors that are relatively insensitive to surrounding magnetic induction.
The idea on which the invention is based is to use three inductive sensors that are connected and fed in suitable manner, thereby reducing the number of sensors, reducing the wiring needed to connect them, reducing the connections and connectors that are required, and limiting the number of magnetic parts that are necessary to hold the sensors mechanically in the stator.
Another object of the invention is to facilitate processing of the output signal for evaluating the radial position of the rotor, and enabling the signal to be used downstream, e.g. to control supporting electromagnets in magnetic bearings for the purpose of keeping the rotor centered.
To achieve these objects, and others, the invention provides a rotary assembly comprising a rotor mounted to rotate in a stator about a mean axis of rotation, the assembly further comprising a detector for detecting the radial position of the rotor within the stator, itself comprising three sensors distributed in the stator at substantially 120xc2x0 from one another about the rotor and at equal distances from the mean axis of rotation; according to the invention:
the sensors are inductive sensors placed facing a segment of the rotor that is made of magnetic material;
a balanced three-phase sinusoidal voltage source having a neutral point feeds the inductive sensors which are wired in a star configuration;
means sense the output voltage between the neutral point of the balanced three-phase voltage source and the connection point common to the star-connected inductive sensors, and deduce therefrom the radial position of the rotor within the stator.
An application of the invention lies in magnetic bearing control systems. Under such circumstances, the rotary assembly comprises at least one magnetic bearing having supporting electromagnets fed by a control circuit driven by the output voltage of the corresponding radial position detector.
In this application, a particularly advantageous solution consists in using a magnetic bearing comprising three supporting electromagnets distributed around the rotor on three axes that are at 120xc2x0 to one another.
In which case, the inductive sensors can advantageously be offset angularly between the angular positions occupied by the supporting electromagnets.
Because there are at least three inductive sensors for a magnetic bearing having three supporting electromagnets, it becomes possible to engage the inductive sensors between the supporting electromagnets of the same magnetic bearing so that the assembly occupies only a small amount of space axially.
A single rotor is preferably provided with two magnetic bearings that are longitudinally spaced apart from each other.
In a first embodiment, the output voltage is processed by signal processing means to produce an amplitude signal and a phase signal relative to the voltage delivered by the balanced three-phase voltage source, the amplitude signal constituting a signal that is an image of the amplitude of the radial displacement of the gravity axis of the rotor relative to the mean axis of rotation, and the phase signal constituting a signal that is the image of the angular position of the gravity axis of the rotor about the mean axis of rotation relative to the inductive sensors. Thus, using only three sensors, two signals are obtained constituting the image of the amplitude and the angular position of the radial displacement of the gravity axis of the rotor within the stator.
In a preferred implementation of this first embodiment, the signal processing means comprise:
an amplitude-measuring circuit connected to the output of the radial position detector and producing an amplitude signal proportional to the amplitude of the output voltage of the radial amplitude detector;
a phase-measuring circuit connected to the output of the radial position detector, and producing a phase signal proportional to the phase offset between the output voltage and one of the three sinewave voltages feeding the star-connected inductive sensors;
digital processing means receiving the amplitude signal and the phase signal, and producing the signals for controlling the feed to the supporting electromagnets seeking to keep the gravity axis of the rotor centered relative to the mean axis of rotation in the stator.
In a second embodiment, the output voltage is processed by signal processing means producing at least two amplitude signals constituting images of respective instantaneous displacement components of the center of gravity of the rotor along two different fixed radial directions of the stator about the mean axis of rotation.
In a preferred implementation of this second embodiment, the signal processing means comprise:
a first amplitude-measuring circuit having a first input connected to the output of the radial position detector and having a second input connected to a first phase of the balanced three-phase sinusoidal voltage source, and outputing a first amplitude signal proportional to the mean value of the output voltage as computed over a sample that is synchronous with said first phase of the balanced three-phase sinusoidal voltage source;
a second amplitude-measuring circuit having a first input connected to the output of the radial position detector and having a second input connected to another phase of the balanced three-phase sinusoidal voltage source, for example the third phase, and outputing a second amplitude signal proportional to the mean value of the output voltage computed during a sample that is synchronous with said other phase of the balanced three-phase sinusoidal voltage source;
digital processing means receiving the two amplitude signals and producing signals for controlling the feed to the supporting electromagnets seeking to keep the gravity axis of the rotor centered relative to the mean axis of rotation within the stator.
A particularly advantageous application of the invention lies in making a rotor and a stator for a vacuum pump.